Optical Coherence Tomography
Optical Coherence Tomography (OCT) is a non-invasive diagnostic technique that renders an in vivo cross-sectional view of the retina. OCT utilizes a concept known as interferometry to create a cross-sectional map of the retina that is accurate to within at least 10-15 microns. OCT was first introduced in 1991 by Huang and colleagues and has found many uses outside of ophthalmology, where it has been used to image certain non-transparent tissues. Due to the transparency of the eye (i.e. the retina can be viewed through the pupil), OCT has gained wide popularity as an ophthalmic diagnostic tool.
Time Domain vs. Spectral Domain vs. Swept Source
At its inception, OCT images were acquired in a time-domain fashion. Time domain systems acquire approximately 400 A-scans per second using 6 radial slices oriented 30 degrees apart. Because the slices are 30 degrees apart, care must be taken to avoid missing pathology between the slices.
Spectral-domain technology, on the other hand, scans approximately 20,000-40,000 A-scans per second. This increased scan rate and number diminishes the likelihood of motion artifacts, enhances the resolution, and decreases the chance of missing lesions. Spectral-domain systems increase the signal-noise ratio by image-averaging multiple B-scans at the same location. Whereas most time domain OCTs are accurate to 10-15 microns, newer spectral domain machines may approach 3-micron resolution. Whereas most time domain OCTs image 6 radial slices, spectral domain systems continuously image a 6mm area. This diminishes the chance of inadvertently missing pathology. Spectral-domain systems typically operate at 800-870 nm wavelengths, although longer wavelengths of 1050-1060 nm are being developed for deeper penetration in the tissue.
In the late 2000s, the advent of enhanced depth imaging (EDI) allowed for better visualization of the choroid and choroidoscleral interface using the spectral domain system. EDI employed the use of image averaging and it set the zero-delay line (ZDL) to adjacent to the choroid.
Swept-source technology, uses a wavelength-sweeping laser and dual-balanced photodetector, allowing for faster acquisition speeds of 100,000-400,000 A-scans per second. This technology uses longer wavelengths of 1050-1060 nm for deeper tissue penetration without the need for EDI. This wavelength provides an axial resolution of about 5.3 um in tissue compared to the approximately 5 um axial resolution of the standard 800 nm wavelength of commercial spectral domain devices. The enhanced axial resolution along with the faster scanning speeds, which allows for greater image averaging, improves image quality and the ability to visualize deeper structures in more detail.
Most recently, both spectral domain and swept source OCT have been used to generate non-invasive non-dye-based OCT angiography (OCTA) images. In brief, OCT angiography uses motion contrast by comparing the decorrelation signal between multiple B-scans obtained at each retinal cross-section to detect blood flow, employing the principle that theoretically only circulating erythrocytes within the retinal capillaries should be moving in the retina.
International Nomenclature for OCT
Suggested the terms band, layer, and zone for the layers of the retina
•The term band refers to the three-dimensional structure of the retinal layers anatomically.
•The term zone describes those regions on OCT whose anatomical correlation is not clearly delineated.
•The RPE (retinal pigment epithelium)/Bruch’s complex is one of the layers ascribed as a zone as they are inseparable owing to interdigitation of cellular structure or tissue.
Artifacts on OCT
Mirror Artifacts (inverted artifacts) in OCT
It occurs when the area of interest crosses the zero delay line and results in an inverted image. In practice, it is especially seen in pathological myopia, retinal detachment, and during the anterior segment OCT.
Vignetting or cut edge artifact:
This occurs when a part of the OCT beam is blocked by the iris or other structures and is characterized by a loss of signal over one side of the image. Some structures may cast an obvious shadow over the retina. These include hemorrhage, pigments, asteroid hyalosis, and vitreous floaters. The shadow by the iris could be avoided by proper dilation of the pupil and moving the OCT machine right or left to capture a clearer image.
Misalignment or off-center artifact or grid decentration artifact:
This occurs when the fovea is not properly aligned during a volumetric scan. Typically it is due to the patient exhibiting poor or eccentric fixation or poor attention. In such cases, the measured central macular thickness (average retinal thickness at the central 1 mm diameter) is wrong as the map is not centered on the fovea or central macula. The knowledge and skills of the technician or the ophthalmic photographer (who takes the OCT image) is important. They should be trained to identify anatomical landmarks and should be guided by the ophthalmologists. Similar misalignment with wrong measurements can happen while measuring central corneal thickness and peripapillary retinal nerve fiber layer thickness. In such cases, the image should be recaptured with proper positioning of the structure of interest.
Out of Range Error or out of register artifact:
The area of interest should be at the center of the image. In the provided image, notice that the outer retina/choroidal image is cut off because of improper positioning of the machine during image acquisition. In such cases, the machine needs to be nearer or far to the patient for proper capture of the image.
Blink artifacts result in partial loss of data due to the momentary blockage of OCT image acquisition during the blink. Blink artifacts are easily recognized as black horizontal bars across the OCT image and macular map.
This occurs when there is movement of the eye during OCT scanning leading to distortion or double scanning of the same area. A slab of the image seems shifted side-wise which is best noticeable as a break in the continuity of retinal vessels.
The software of the OCT machine automatically detects the border of the inner retina (internal limiting membrane) and the outer retina to calculate the retinal thickness map. The definition of outer retinal margin varies according to the imaging device (inner segment-outer segment junction for Stratus, inner part of RPE for Copernicus and Topcon 3D-OCT 1000, middle of RPE for Cirrus, outer part of RPE for Optovue RTVue 100, and Bruch’s membrane for Spectralis). The software automatically calculates the distance between the inner and outer margin of the neurosensory retina to give the thickness values in the ETDRS grid of the retinal thickness map. In many cases including vitreoretinal interface disorders and diseases of outer retina/choroid (including age-related macular degeneration), this automatic detection may be defective (shifted vertically) resulting in wrong thickness values in the map. Both the outer and inner retinal border lines may shift vertically without causing a change in the macular thickness map values. This is called the segmentation shift.
OCT is useful in the diagnosis of many retinal conditions, especially when the media is clear. In general, lesions in the macula are easier to image than lesions in the mid and far periphery. OCT can be particularly helpful in diagnosing:
- Macular hole
- Macular pucker/epiretinal membrane
- Vitreomacular traction
- Macular edema and exudates
- Detachments of the neurosensory retina
- Detachments of the retinal pigment epithelium (e.g. central serous chorioretinopathy or age-related macular degeneration)
- Choroidal tumors
In some cases, OCT alone may yield the diagnosis (e.g. macular hole). Yet, in other disorders, especially retinal or choroidal vascular disorders, it may be helpful to order additional tests (e.g. fluorescein angiography or indocyanine green angiography).
The OCT scan of a pseudohole will reveal an epiretinal membrane with no loss of retinal layers. Foveal pit is narrow and vertical. The inner retina is thickened around the fovea. The perifoveal thickness may be increased. The term “pseudohole” reflects the fact that although this looks like a macular hole, it is not a hole in the retina.
Ectopic inner foveal layer (EIFL)
EIFL is characterized by the presence of continuous hyporeflective and hyperreflective bands extending from the inner nuclear layer and inner plexiform layer across the foveal region.
Inner retinal hyper-reflectivity (Diffuse)
In CRAO (central retinal arterial occlusion), inner retinal layers appear as a hyperreflective band which may be swollen.
Focal hyperreflectivity in Hyperreflective Foci (HRF) :
These are typically dot-like or round regular lesions seen in all the retinal layers and choroid, less than 30 microns in size. They typically lack back-shadowing and do not have a representative visible fundus lesion.
DRIL ( Disorganized retinal inner layers ) :
DRIL is identified when the boundaries of the ganglion cell layer, inner plexiform layer, inner nuclear layer, and outer plexiform layer cannot be identified and demarcated. It is a potential biomarker for poor visual prognosis in DME.
Cystoid macular edema (CME) : Cystoid macular edema can be seen on OCT scans as multiple circular or oval hyporeflective spaces in the retina, indicating intraretinal edema.
This is characterized by intraretinal splitting at fovea with vertical tissue bridges connecting the inner and outer retina. Fluorescein angiography does not show petaloid macular leak as is seen in CME. Visual acuity may be disproportionately good despite the increase in the central macular thickness.
There is a splitting of retinal layers at the outer plexiform layer usually in the inferotemporal peripheral fundus in elderly hyperopic patients. On the contrary, retinal detachment shows separation of the neurosensory retina and the retinal pigment epithelium.
Important findings on OCT:
Pearl necklace sign :
Hyperreflective dots are arranged in a contiguous ring around the inner wall of cystoid spaces in the outer plexiform layer of the retina. It is usually seen in exudative macular diseases.
Paracentral Acute Middle Maculopathy (PAMM) :
It is an OCT finding characterized by a parafoveal hyperreflective band at the level of the inner nuclear layer.
IS-OS (inner segment-outer segment junction, now called ellipsoid zone or EZ) Loss
Disruption of the IS=OS line has been demonstrated to correlate with retinal function loss in several retinal disorders and is considered a useful indicator of photoreceptor integrity and predictor of visual function.
ILM (internal limiting membrane) Drape Sign in IPFT (idiopathic parafoveal telangiectasia or macular telangiectasia type 2)
The ILM drape sign occurs when a thin membrane overhangs this central hyporeflective lesion at the base of the fovea. The central macular thickness may be reduced. It is seen in macular telangiectasia 2.
Outer Retinal Tubulations ( ORT) :
ORT is a hyporeflective area surrounded by hyper-reflective band in the outer nuclear layer. It comprises interconnecting tubes containing degenerating photoreceptors, almost exclusively cones and Müller cells.
Subretinal hyperreflective material (SHRM) in CNVM :
This SD-OCT feature is identified as hyperreflective material located between the neurosensory retina and retinal pigment epithelium (RPE).
Subretinal hyporeflectivity in SMD (Serous macular detachment)
Serous macular detachment (SMD) with cystoid diabetic macular edema (DME), shows retinal elevation with an optically clear space between the sensory retina and the retinal pigment epithelium.
Pigment epithelial detachment PED
Serous PEDs appear as well-demarcated, abrupt elevations of the RPE with a homogenously hyporeflective sub-RPE space.
Idiopathic polypoidal choroidal vasculopathy IPCV
OCT features of IPCV are multiple PEDs, sharp PED peak, PED notch, and rounded sub-RPE hyporeflective area.
Choroidal NEVUS :
It is seen as Hyporeflectivity in the anterior surface of the choroid.
CONTOUR abnormalities :
Dome shaped maculopathy : It is an anterior convex protrusion of the macula towards the vitreous cavity seen on OCT. It is associated with high myopia and posterior staphyloma.
It is seen as an outward globe bulge, resulting in a deep concave B-scan OCT and distorted retinal structures.
Bacillary layer detachment : a photoreceptor splitting detachment
Note the hyperreflective material along the outer retinal surface. A thin band at the base of cystic detachment is continuous with the adjacent ellipsoid band and the external limiting membrane.
Focal Choroidal Excavation
It is defined as an area of concavity in the choroid detected on OCT. These are mostly present in the macular region without evidence of accompanying scleral ectasia or posterior staphyloma.
The presence of subfoveal perfluorocarbon liquid (PFCL) after vitreoretinal surgery can cause the appearance of cystoid foveal edema.
Other OCT Signs :
Dipping (tenting down) sign may be observed in some acute CSCR patients.
It is characterized by dipping or tenting at the outer surface of the detached neurosensory retina due to hyperreflective material accumulation such as subretinal fibrin or fibrinous exudate connecting the detached neurosensory retina and RPE at its opposite.
OMEGA Sign :
OMEGA sign is an omega-shaped disorganization of inner retinal layers bounded posteriorly by the outer plexiform layer.
Omega sign is a characteristic feature of macular CHRRPE and may help to distinguish macular CHRRPEs from ERMs.
ONION Sign :
The "onion sign" refers to layered hyper-reflective bands in the sub-RPE space usually associated with chronic exudation from type 1 neovascularization in patients with AMD.
Cotton Ball sign :
It is a roundish or diffuse highly reflective region observed between the photoreceptor inner segment/outer segment junction line and the cone outer segment tip line at the center of the fovea.
This highly reflective region is a characteristic sign observed in the OCT images of eyes with VMT and ERM.
BRUSH BORDER PATTERN :
This is an irregular, serrated, and thicker appearance of the detached neurosensory retina seen in CSCR.
It is due to the accumulation of waste products in the photoreceptor outer segment on the outer surface of the detached neurosensory retina over subretinal fluid.
Various Classifications based on OCT
Macular Hole classification:
Diabetic macular edema classification:
|OCT CLASSIFICATION OF DME||OCT features|
|1||Diffuse retinal thickening|
|2||Cystoid macular edema|
|3||Posterior hyaloid traction|
|4||Subretinal fluid/serous retinal detachment|
|5||Tractional retinal detachment|
Ectopic inner foveal layer Classification:
|Ectopic Inner Foveal Layer classification|
|Stage 1 ERM||Epiretinal membrane with foveal preservation|
|Stage 2 ERM||ERM
Loss of foveal depression
Thickening of ONL
|Stage 3 ERM||ERM
Loss of foveal depression
Continuous & clearly identified EIFL
|Stage 4 ERM||ERM with loss of foveal depression
EIFL with anatomy & identification completely lost
OCT is gaining increasing popularity when evaluating optic nerve disorders by accurately and reproducibly evaluating the retinal nerve fiber layer and ganglion cell layer thickness:
- Optic neuritis
- Non-glaucomatous optic neuropathies
- Alzheimer's disease
Anterior segment OCT utilizes higher wavelength light than traditional posterior segment OCT. This higher wavelength light results in greater absorption and less penetration. In this fashion, images of the anterior segment (cornea, anterior chamber, iris, and angle) can be visualized.
Because OCT utilizes light waves (unlike ultrasound which uses sound waves) media opacities can interfere with optimal imaging. As a result, the OCT will be limited in the setting of vitreous hemorrhage, dense cataracts, or corneal opacities.
As with most diagnostic tests, patient cooperation is a necessity. Patient movement can diminish the quality of the image. With newer machines, acquisition time is shorter which may result in fewer motion-related artifacts.
The quality of the image is also dependent on the operator of the machine. Early models of OCT relied on the operator to accurately place the image over the desired pathology. When serial images were acquired over time (e.g. during treatment for AMD with anti-VEGF therapy), later images could be taken that were off-axis compared to earlier images. Newer technologies, such as eye-tracking equipment, limit the likelihood of acquisition error.
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- Markan, A., Agarwal, A., Arora, A., Bazgain, K., Rana, V., & Gupta, V. (2020). Novel imaging biomarkers in diabetic retinopathy and diabetic macular edema. Therapeutic advances in ophthalmology, 12, 2515841420950513. https://doi.org/10.1177/2515841420950513